Plants are exposed to numerous denizens of their environment, including bacteria, viruses, fungi, and nematodes. Although many of the interactions between these organisms and plants, particularly via the roots of the plants, are beneficial, many of the interactions are harmful to the plants. The decimation of agricultural crops, ornamental plants, and other plants by diseases caused by plant pathogens, particularly fungal pathogens, is a worldwide problem that has enormous economic impact.
Damage to plants is caused by pathogens of multiple genera. These genera include Alternaria, Ascochyta, Aspergillus, Botrytis, Cercospora, Colletotrichum, Diplodia, Erwinia, Erysiphe, Fusarium, Gaeumanomyces, Helminthosporium, Macrophomina, Magnaporthe, Mycosphaerella, Nectria, Peronospora, Phoma, Phymatotrichum, Phytophthora, Plasmopara, Podosphaera, Pseudomonas, Puccinia, Puthium, Pyrenophora, Pyricularia, Pythium, Rhizoctonia, Scerotium, Sclerotinia, Septoria, Thielaviopsis, Uncinula, Venturia, Verticillium, and Xanthomonas. 
Many chemical compounds have been developed to combat these various pathogens. Examples of chemical antifungal agents include polyoxines, nikkomycines, carboxyamides, aromatic carbohydrates, carboxines, morpholines, inhibitors of sterol biosynthesis, and organophosphorus compounds (Worthington and Walker, 1983; U.S. Pat. No. 5,421,839). The activity of these compounds is typically limited to several species. As a consequence of the large number and diversity of pathogenic fungi, these compounds have not provided an effective solution to limiting infections in plants.
An alternative approach to controlling pathogenic infections in plants involves exploiting the natural defense mechanisms of plants to confer resistance. Many plants have developed natural resistance to some pathogens. However, resistance may be limited to certain genera of pathogens, or crops of agronomic interest may not exhibit sufficient resistance. Thus, natural plant defenses often do not provide sufficient protection against pathogens. By broadening the spectrum of pathogen defense or strengthening the defense response, it may be possible to enhance existing resistance mechanisms and promote pathogen defense in otherwise susceptible plants.
When present and active, the natural defense mechanisms of plants are highly effective in preventing pathogen colonization and disease. Resistance is multi-tiered, with passive and active, constitutive and inducible elements (Baker et al., 1997; Keen, 1990; Ryals et al., 1996). Inducible defense can be activated through the action of plant recognition of a pathogen determinant, or elicitor, to trigger a localized cell death or hypersensitive response (HR) at the site of pathogen attack (Dixon et al., 1994). This localized apoptotic cell death is often mediated by resistance genes (R-genes) that recognize a specific, cognate “avirulence” product in the pathogen (Greenberg, 1997). The local perception of pathogen attack is conveyed to distant tissues via a transmissible signal that involves salicylic acid (SA), further activating gene expression and conditioning a state known as systemic acquired resistance (SAR; Ryals et al., 1996; Sticher et al., 1997). It has subsequently been found that resistance can be expressed near the region of pathogen attack, as local acquired resistance, or can be induced systemically, depending on triggering signal and plant species. Thus the systemic and local responses collectively are referred to as acquired resistance (AR). Establishment of AR is a powerful line of plant defense because it can provide broad-spectrum resistance against viral, bacterial, and fungal challenges that would otherwise cause disease (Cameron et al., 1994; Gorlach et al., 1996; Ryals et al., 1996). The AR response triggers the transcriptional activation of a suite of genes encoding pathogenesis-related (PR) proteins. Included among these are hydrolases, cell-wall strengthening proteins, proteins involved in oxidative burst, the combination of which are believed to promote heightened resistance (Sticher et al., 1997).
Biochemical and genetic analyses have identified genes and molecular signals associated with acquired resistance. The Npr1/Nim1 gene plays a key regulatory role in the AR defense in Arabidopsis against a broad spectrum of fungal and bacterial pathogens (Cao et al., 1994; Cao et al., 1997; Delaney et al., 1995; Ryals et al., 1997; WO 98/06748; WO 94/16077; WO 98/26082). Mutant npr1 plants induce normal HR and accumulate SA after avirulent pathogen challenge, but they fail to accumulate PR proteins or activate the AR response, suggesting that this protein functions in the pathway downstream from salicylic acid (Cao et al., 1994; Cao et al., 1997; Delaney et al., 1995). Features of the Npr1 protein suggest a role as a transcriptional regulator and include motifs such as ankyrin repeats, implied in protein-protein interactions; nuclear localization signals; putative phosphorylation sites; and homology with IFκB, a transcriptional regulator in mammalian systems (Cao et al., 1997; Ryals et al., 1997). Nuclear translocation of activated Npr1 has been demonstrated, strengthening its likely role in transcriptional regulation (WO 98/06748). The central importance of Npr1 in dicots was further substantiated by transgenic overexpression of the cloned gene, which led to heightened disease resistance in Arabidopsis against both fungal and bacterial pathogens (Cao et al., 1998; WO 98/06748).
Although the bulk of AR research has defined the pathway in dicotyledonous plants, monocotyledonous plants, such as wheat, rice, and barley, have an inducible pathway that protects against pathogen attack (Hwang and Heitefuss, 1982; Kmecl et al., 1995; Schweizer et al., 1989; Smith and Metraux, 1991). Acquired resistance can be conditioned by different external stimuli, including avirulent pathogen challenge (Manandhar et al., 1998; Schaffrath et al., 1997), pathogen elicitor exposure (Jin et al., 1997; Schaffrath et al., 1995; Waspi et al., 1998), and chemical treatments, including application of SA or SA analogs, such as 2,6-dichloroisonicotinic acid (INA) or benzo(1,2,3) thiodiazole-7-carbothioic acid S-methyl ester (BTH) (Gorlach et al., 1996; Kessman et al., 1994; Kogel et al., 1994; Manandhar et al., 1998; Schaffrath et al., 1997; Watanabe et al., 1979). Given the inducibility of the AR pathway by the same classes of activating compounds in monocot and dicot plants, there is likely to be partial conservation of signaling pathways, as subsets of PR genes appear to be induced in both groups (Morris et al., 1998). However, studies also point to marked differences in monocots, with inducers of AR revealing new pathways that are tied to new classes of PR genes (Gorlach et al., 1996; Schaffrath et al., 1997). In monocots, induced acquired resistance is broad-spectrum, extending to fungal and bacterial pests, irrespective of pathogen race, with activated resistance persisting for weeks to months. Thus, manipulation of the AR pathway in monocot plants may promote resistance to pathogens for which there exists no genetic source of resistance.
Thus, there is a need to identify genes from monocotylendonous crops, such as wheat and rice, that may play key roles in disease defense. Overexpression of these genes in transgenic plants is expected to enhance the level of disease resistance against certain microbial pathogens. It has, therefore, been discovered that a gene isolated from rice, designated Nph1, and a gene isolated from wheat, designated Nph2, are induced by chemical elicitors known to stimulate AR. Activation of AR and induced expression of Nph1 and Nph2 therefore is expected to protect wheat and rice against biotrophic pathogens. Transgenic overexpression of Nph1 and Nph2 should condition a stronger AR upon pathogen challenge, thus promoting more effective disease protection.